In the operation of diesel engines that are adapted for utilizing alternative fuels, the use of a glow plug to beneficially assist the ignition of the non-autoignitable fuel during start-up as well as during operation is well known. It is also well known that such glow plugs have a less than desirable service life owing to the harsh environment in the combustion chamber due to elevated temperatures. Particularly, where the glow plug is formed of a ceramic material, such as silicon nitride, the service life of the glow plug is further reduced due to thermal stresses, oxidation and corrosion. The operating longevity of a silicon nitride glow plug is further compromised when it is utilized in a diesel engine that is burning fuel other than diesel fuel.
When a silicon nitride glow plug is utilized to assist in the ignition of non-autoignitable fuels, at the elevated temperatures needed to sustain fuel combustion, the silicon nitride undergoes severe corrosion and erosion due in part to the presence of impurities such as sodium, calcium, magnesium and sulfur introduced by the fuel and the lubrication oil. At Thigh temperatures, these impurities react with the normally stable SiO.sub.2 layer on the silicon nitride surface to form compounds such as Na.sub.2 SO.sub.4 having a lower melting temperature than silicon nitride, which are progressively eroded away by fuel and air spray.
It is desirable to provide the surface of a ceramic glow plug with a protective coating that is not attacked by the impurities in the combustion environment and thus inhibits the corrosion and/or erosion mechanism. It is also desirable that the protective coating have very good adhesion to the glow plug surface. It is further desirable that the protective coating have uniform continuity across the surface of the glow plug to provide uniform corrosion and erosion protection.
To solve the problems described above, this invention resides in a process for coating preselected portions of a silicon nitride glow plug with a corrosion and erosion inhibiting material such as tantalum oxide, which has a compatible coefficient of thermal expansion as silicon nitride. The process embodied in the invention results in very good adhesion of tantalum oxide to the silicon nitride surface of the glow plug. The process embodied in the invention also results in good continuity of tantalum oxide across the surface of silicon nitride, resulting in uniform corrosion inhibiting characteristics. The tantalum oxide coating deposited by the process of the present invention desirably seals the glow plug from the detrimental environment generated by the use of alternative fuels. The present invention is directed to overcome one or more of the problems as set forth above.